Full 3D Simulation Research Articles

A sliced-3D approach to finite-difference time-domain modeling by optimizing perfectly matched layers

Finite-difference time-domain forward modeling is often used to gain a more quantitative understanding of the interactions between electromagnetic fields and targets. To undertake full 3D simulations, the computational demands are challenging, so simulations are often undertaken in two dimensions, in which assumptions in the propagation of electromagnetic fields and source type can result in errors. We have developed the concept of a sliced-3D simulation, wherein a thin slice of a 3D domain with strictly 2D geometry is used to minimize computational demands while obtaining synthetic waveforms that contain full 3D propagation effects. This approach requires optimization of perfectly matched layer (PML) boundary condition parameters so as to minimize the errors associated with the source being located close to the boundary, and as a result of grazing-incident angle wave conversion to evanescent energy. We explore the frequency dependence of PML parameters, and we establish a relationship between complex frequency stretching parameters and effective wavelength. The resultant parameter choice is shown to minimize propagation errors in the context of a simple radioglaciological model, in which 3D domains may be prohibitively large, and for a near-surface cross-borehole survey configuration, a case in which full-waveform inversion may typically be used.

Understanding Heat Transfer During the Secondary Drying Stage of Freeze Drying: Current Practice and Knowledge Gaps

Currently, there is a lack of robust models for secondary drying with comparable accuracy and flexibility as primary drying models. In order to better understand heat transfer during secondary drying, sucrose and mannitol solutions were freeze-dried in vials in a lab-scale lyophilizer under various drying conditions. Several distinct thermal characteristics for secondary drying were experimentally observed: (1) the vial heat transfer coefficient can change significantly between primary and secondary drying due to the change in water vapor content in the freeze dryer; (2) the thermal mass of the vial plays a major role in determining the cake temperature as roughly 95% of the heat supplied is absorbed by the vial walls. From a theoretical perspective, three different models of secondary drying were examined with varying degrees of complexity (full 3D simulation, 1D-averaged equations, and lumped-capacitance 0D approach). In these models, the desorption of bound water is treated as a one-way coupling with temperature. It is found that although a simple lumped-capacitance approach can capture many of the vital features of cake temperature and moisture profile, near quantitative agreement with experiments can be made by employing a 1D-averaged equation approach, where the effective thermal conductivities of the vial are determined by thermal circuits.

Symmetric and asymmetric shocked gas jets for laser-plasma experiments.

Shocks in supersonic flows offer both high density and sharp density gradients that are used, for instance, for gradient injection in laser-plasma accelerators. We report on a parametric study of oblique shocks created by inserting a straight axisymmetric section at the end of a supersonic "de Laval" nozzle. The effect of different parameters, such as the throat diameter and straight section length on the shock position and density, is studied through computational fluid dynamics (CFD) simulations. Experimental characterizations of a shocked nozzle are compared to CFD simulations and found to be in good agreement. We then introduce a newly designed asymmetric shocked gas jet, where the straight section is only present on one lateral side of the nozzle, thus providing a gas profile well adapted for density transition injection. In this case, full-3D fluid simulations and experimental measurements are compared and show excellent agreement.

One- and three-dimensional coupling flow simulations of pumped-storage power stations with complex long-distance water conveyance pipeline system

This study developed a one-dimensional and three-dimensional (1D–3D) coupling transient flow simulation method to investigate the effect of nonlinear fluctuations of pressures and hydraulic thrusts on the impeller and reveal their underlying flow mechanism during a combined operation mode, comprising two parallel pump-turbines, in a complex water conveyance pipeline system at an actual pumped-storage power station. Experimental verification suggested that the 1D–3D coupling method could accurately simulate the pressure fluctuations and rotational speed of the impeller. Additionally, three combined operation modes consisting of two parallel pump-turbines in a complex hydraulic pipeline system at an actual pumped-storage power station were simulated to test the performances of the developed 1D–3D coupling method. Test results suggested that the proposed method can successfully capture the nonlinear fluctuations of pressures and hydraulic thrusts on the impeller and water hammer phenomena. Additionally, it can reproduce the local and global backflow in the impeller, which induce fluctuations in the pressures and hydraulic thrusts. This study demonstrated that the 1D–3D coupling flow simulation method can provide more transient flow in a pumped-storage power station, possessing a complex long-distance hydraulic pipeline system, using lower computational cost than the conventional pure 1D and full 3D simulation method.

Systematic description of wind-driven protoplanetary discs

Aims. Planet-forming discs are believed to be very weakly turbulent in the regions outside of 1 AU. For this reason, it is now believed that magnetised winds could be the dominant mechanism driving accretion in these systems. However, currently, no self-consistent approach can describe discs that are subject to a magnetised wind in a way similar to the α disc model. In this article, I explore the parameter space of wind-driven protoplanetary discs in a systematic manner and present scaling laws that can be used in reduced models in a similar way to α disc models. Methods. I computed a series of self-similar wind solutions, assuming that the disc is dominated by ambipolar and Ohmic diffusion. These solution were obtained by searching for stationary solutions in the finite-volume code PLUTO using a relaxation method and continuation. Results. Self-similar solutions are obtained for values of plasma β ranging from 102 to 108 for several Ohmic and ambipolar diffusion strengths. Mass accretion rates of about 10−8 M⊙ yr−1 are obtained for the poloidal field strength β = O(104) or equivalently, 1 mG at 10 AU. In addition, the ejection efficiency is always close to 1, implying that wind mass-loss rate can be higher than the inner mass accretion rate when the wind-emitting region is large. The resulting magnetic lever arms are typically lower than 2, possibly reaching 1.5 in the weakest field cases. Remarkably, the mean transport properties (accretion rate and mass-loss rate) mostly depend on the field strength and much less on the disc diffusivities or surface density. The disc internal structure is nevertheless strongly affected by Ohmic resistivity, strongly resistive discs being subject to accretion at the surface while ambipolar only models lead to mid-plane accretion. Finally, I provide a complete set of scaling laws and semi-analytical wind solutions, which can be used to fit and interpret observations. Conclusions. Magnetised winds are unavoidable in protoplanetary discs as soon as they are embedded in an ambient poloidal magnetic field. Very detailed disc microphysics are not always needed to describe them, and simplified models such as self-similar solutions can capture most of the physics seen in full 3D simulations. The remaining difficulty to set up a complete theory of wind-driven accretion lies in the transport of the large-scale field, which remains poorly constrained and is not well understood.

A non-linear finite element connector model with friction and plasticity for the simulation of bolted assemblies

Accurate simulation of bolted joints is not always consistent with industrial requirements, since numerous nonlinearities in the vicinity of the bolt can lead to overly expensive calculations. For this reason, commercial finite element (FE) codes preferentially use substitutes for bolts, such as simplified models or connectors. In this paper, a nonlinear FE connector with its identification methodology is proposed to model the behavior of a bolted assembly. The connector model is based on practical design parameters, such as bolt preload, friction coefficients, or plastic material parameters. The connector is based on a separation of the different phenomena governing the macroscopic behavior of a bolted assembly. The axial behavior of the connector reflects the preload effect and the axial stiffness of the assembly, while the tangential behavior of the connector takes into account the friction phenomena that occur in the vicinity of the bolt. At the same time, a plastic behavior law with isotropic hardening for the bolt is proposed. The identification of the connector parameters is performed on a generic elementary one-bolt assembly. The connector is implemented in ABAQUS through a user-element subroutine. Comparisons of quasi-static responses between large-scale full 3D calculations and simulations with connectors on different bolted assemblies are provided. The results obtained are very similar, while observing a significant reduction in CPU time.

Laser-driven ion acceleration from near-critical Gaussian plasma density profile

In this paper, we report on multiple phases of efficient laser-driven ion acceleration from near-critical density plasma of Gaussian density profile. Tracking of high-energy accelerated ions in multidimensional particle-in-cell simulations reveals the development of accelerating fields affecting the particles and the contribution of each acceleration phase to final ion energies. While the acceleration of ions occurs in a short time interval when a steep (infinite) density gradient is present, the accelerating field affecting the most energetic ions has unexpected local maxima about 50 fs after the moment when ultrashort (30 fs) laser pulse completely left the target with smooth density gradients. This field can be attributed to the apex of electron filament created behind the transmitted laser pulse. Full 3D simulation confirms the observations in 2D simulations in terms of ion acceleration mechanisms. However, it shows a substantial reduction of maximum achievable ion energies and a larger angular spread of accelerated ions compared with 2D approach, which demonstrates the necessity of using computationally demanding full 3D geometry for similar numerical studies.

Thermal effects in field electron emission from idealized arrangements of independent and interacting micro-protrusions

Modelling studies of thermo-field electron emission (TFE) from protrusions at a cathode surface usually use simulations in 2D axial symmetry. Indeed, time-dependent simulations in 3D are very demanding in computation time. Often, 3D simulations have been restricted to stationary pure field electron emission to account for the drastic current decrease caused by electric field screening when the emitters are close. Little interest has therefore been granted to the heat exchanges occurring between nearby emitters. Although the temperature is a second-order parameter in TFE compared to the electric field, thermal effects become non-negligible in high current density regimes, where self-heating is well established. The present study focuses on the thermal effects occurring during the TFE from micro-protrusions. Our model considers a DC voltage but solves in time the temperature evolution coupling the heat equation and the current continuity equation. The protrusions are modelled as hemiellipsoids with 2D axial symmetry. Emission enhancement due to the increase of the temperature in the thermo-field regime compared to the pure field regime is detailed as a test case for isolated protrusions. Then, full 3D simulations are used to investigate the thermal coupling between multiple neighbouring protrusions via their outwards heat fluxes inside the cathode. The results show a higher current increase due to thermal coupling for dome-like protrusions with a low field enhancement factor. The current increases up to 13% of the total current for aspect ratios of 1, but this value is reached for an extreme applied electric field, hardly reachable in experiments. For sharper protrusions with higher field enhancement, the interaction range through the cathode being shorter, the thermal coupling is suppressed by electrostatic screening. Nevertheless, in arrangements of densely distributed field emitter, when the screening is compensated by a higher voltage, our model predicts the possibility of a moderate but noticeable thermal coupling even for sharp protrusions: a parametric study indicates up to 14.5% of the emitted current being caused by a thermal coupling through the cathode bulk, for protrusions with an aspect ratio of 10 under a fixed applied electric field of 0.4 GV m−1 in DC mode.

Full Loop Numerical Simulation of Fluidization Characteristics at Cyclone and Furnace of 110 MW CFB

Circulating fluidized bed (CFB) boiler has strong points with its recognition when operating with low quality coal contrast to pulverized coal boiler. Boiler load could be improved by increasing coal input and air ratio combustion adjustment. That is why ratio combustion between secondary and primary air is one of valuable parameters influencing fluidization of CFB Boiler. CFB boiler is consist mainly of a furnace, two cyclones, and forced loop pipes, where cyclones are boiler parts that were indicated to have high velocity fluidization up to 30 m/s. Full loop 3D simulation of CFB with focusing on the furnace and cyclone by using computational fluid dynamic (CFD) program was implemented using Eulerian multiphase model to investigate sand volume of fraction, air, and sand velocity including pressure distribution around Cyclone. Several air ratio combustions between primary and secondary air were simulated such as 55%-45%, 50%-50% with 63%, and 100% load variations. It was shown that operation with air combustion ratio 50%-50% and 55%-45% leads to good fluidization at 63% load, while 100% and 110% fluidization will cause an abundant sand entered to inlet cyclone and induce higher sands and air velocity.

Energy and Communication Efficient FSS for Equivalent Model of Hard-Coated Energy Saving Glass

An energy and communication efficient FSS is presented for equivalent model of ESG with hard coating. An FSS with second order band-pass response has been realized by full wave 3D simulations and results have been imitated using an equivalent circuit model to calculate the value of lumped elements. The design is composed of three cascaded FSSs with two glass layers of 6 mm each in between. Due to this topology the transmission of useful RF/microwave signals has been improved for a wider bandwidth, increased thermal insulation, stable frequency and polarization at oblique incidence and security for WLAN at 2.45 GHz.

Interaction of Ultraintense Radially-Polarized Laser Pulses with Plasma Mirrors

We present experimental results of vacuum laser acceleration (VLA) of electrons using radially polarized laser pulses interacting with a plasma mirror. Tightly focused radially polarized laser pulses have been proposed for electron acceleration because of their strong longitudinal electric field, making them ideal for VLA. However, experimental results have been limited until now because injecting electrons into the laser field has remained a considerable challenge. Here, we demonstrate experimentally that using a plasma mirror as an injector solves this problem and permits to inject electrons at the ideal phase of the laser, resulting in the acceleration of electrons along the laser propagation direction while reducing the electron beam divergence compared to the linear polarization case. We obtain electron bunches with few-MeV energies and a 200 pC charge, thus demonstrating for the first time electron acceleration to relativistic energies using a radially polarized laser. High-harmonic generation from the plasma surface is also measured and provides additional insight into the injection of electrons into the laser field upon its reflection on the plasma mirror. Detailed comparisons between experimental results and full 3D simulations unravel the complex physics of electron injection and acceleration in this new regime: we find that electrons are injected into the radially polarized pulse in the form of two spatially-separated bunches emitted from the p-polarized regions of the focus. Finally, we leverage on the insight brought by this study to propose and validate a more optimal experimental configuration that can lead to extremely peaked electron angular distributions and higher energy beams.

Head‐only21MHzradiofrequency exposure system design and implementation

In this work a 21 MHz radiofrequency (RF) exposure system was designed, constructed and evaluated as a testing platform for MRI device safety. The exposure system consisted of a birdcage coil powered in quadrature by a power system consisting of a hybrid coupler, a power splitter, a phase shifter and two baluns. The birdcage coil was designed as a low-pass 16 rung coil. Both the coil and the power system were fabricated as printed circuit boards with a copper thickness of 205.7 μm and a G10 substrate thickness of 581.7 μm. The birdcage was assembled by wrapping the copper clad G10 inside of an acrylic tube. A shield for the coil was created as a cylindrical ground plane with a diameter of 39.7 cm. The birdcage and the power system were simulated in ADS and EMPro, Keysight Technologies Inc. both as lumped elements and using full wave 3D simulations. Simulation results were compared to measurements of the produced components to validate each component and entire system. In addition, a 3D magnetic field probe was developed to measure and compare the fields to analytical and numerical calculations.

Waves and structure interaction using multi-domain couplings for Navier-Stokes solvers in OpenFOAM®. Part II: Validation and application to complex cases

In this work, we present several applications of the 2D-3D multi-domain couplings for Navier-Stokes models developed and validated in its companion (Di Paolo et al., submitted). The methodology is used to carry out some relevant simulations which include long regular and irregular waves, solitary wave propagation on a shallow foreshore, focused wave group transformation on a planar beach, wave impact on a cylinder and finally, the numerical twin of a complex laboratory experiment to analyse the performance of a perforated breakwater under wave action.Results agree well with the full 3D simulations and laboratory experiments and demonstrate the feasibility of using the 2D-3D coupled methodologies presented in Part I to successfully replace full 3D modelling. For all the cases considered, the application of coupled methodologies have resulted in a drastic reducing of the computational time without decreasing the accuracy of the full solution.

The geometry and dynamical role of stellar wind bubbles in photoionized H ii regions

ABSTRACT Winds from young massive stars contribute a large amount of energy to their host molecular clouds. This has consequences for the dynamics and observable structure of star-forming clouds. In this paper, we present radiative magnetohydrodynamic simulations of turbulent molecular clouds that form individual stars of 30, 60, and 120 solar masses emitting winds and ultraviolet radiation following realistic stellar evolution tracks. We find that winds contribute to the total radial momentum carried by the expanding nebula around the star at 10 per cent of the level of photoionization feedback, and have only a small effect on the radial expansion of the nebula. Radiation pressure is largely negligible in the systems studied here. The 3D geometry and evolution of wind bubbles is highly aspherical and chaotic, characterized by fast-moving ‘chimneys’ and thermally driven ‘plumes’. These plumes can sometimes become disconnected from the stellar source due to dense gas flows in the cloud. Our results compare favourably with the findings of relevant simulations, analytic models and observations in the literature while demonstrating the need for full 3D simulations including stellar winds. However, more targeted simulations are needed to better understand results from observational studies.

High resolution parameter study of the vertical shear instability

ABSTRACT Theoretical models of protoplanetary discs have shown the vertical shear instability (VSI) to be a prime candidate to explain turbulence in the dead zone of the disc. However, simulations of the VSI have yet to show consistent levels of key disc turbulence parameters like the stress-to-pressure ratio α. We aim to reconcile these different values by performing a parameter study on the VSI with focus on the disc density gradient p and aspect ratio h = H/R. We use full 2π 3D simulations of the disc for chosen set of both parameters. All simulations are evolved for 1000 reference orbits, at a resolution of 18 cells per h. We find that the saturated stress-to-pressure ratio in our simulations is dependent on the disc aspect ratio with a strong scaling of α∝h2.6, in contrast to the traditional α model, where viscosity scales as ν∝αh2 with a constant α. We also observe consistent formation of large scale vortices across all investigated parameters. The vortices show uniformly aspect ratios of χ ≈ 10 and radial widths of approximately 1.5H. With our findings we can reconcile the different values reported for the stress-to-pressure ratio from both isothermal and full radiation hydrodynamics models, and show long-term evolution effects of the VSI that could aide in the formation of planetesimals.

Gravitational Waves from Neutrino Emission Asymmetries in Core-collapse Supernovae

Abstract We present a broadband spectrum of gravitational waves (GWs) from core-collapse supernovae (CCSNe) sourced by neutrino emission asymmetries for a series of full 3D simulations. The associated GW strain probes the long-term secular evolution of CCSNe and small-scale turbulent activity and provides insight into the geometry of the explosion. For nonexploding models, both the neutrino luminosity and the neutrino gravitational waveform will encode information about the spiral SASI. The neutrino memory will be detectable for a wide range of progenitor masses for a galactic event. Our results can be used to guide near-future decihertz and long-baseline GW detection programs, including aLIGO, the Einstein Telescope, and DECIGO.

Computational investigation of annular flow condensation in microgravity with two-phase inlet conditions

Condensation heat exchangers contribute significantly to the size of the thermal management system, and their implementation in future spacecraft will require a better understanding of the underlying heat transfer phenomena. In this study, we computationally investigate flow condensation in microgravity with two-phase inlet conditions utilizing the VOF multi-phase model and a 2-dimensional axisymmetric domain. The model is validated utilizing experiments from prior microgravity flow condensation tests conducted onboard a parabolic flight. A control-volume-based theoretical model is utilized to estimate the inlet vapor fraction and inlet phase velocity boundary conditions. The computational model predicts complex flow behavior occurring at the two-phase interface during condensation, but the model suffers from some liquid accumulation in the vapor region. Local condensation heat transfer is under-estimated in the inlet section, and over-estimated in the exit section, compared with the experimental measurements. Mean condensation heat transfer coefficients compare well to experiments. Results show a need for full 3D simulations of flow condensation to improve predictions of liquid entrainment and liquid deposition phenomena that significantly influence local heat transfer coefficients predictions

Can We Trust MHD Jump Conditions for Collisionless Shocks?

Abstract When applied to compute the density jump of a shock, the standard magnetohydrodynamic (MHD) formalism assumes (1) that all the upstream material passes downstream, together with the momentum and energy it carries, and (2) that pressures are isotropic. In a collisionless shock, shock-accelerated particles going back and forth around the front can invalidate the first assumption. In addition, an external magnetic field can sustain stable pressure anisotropies, invalidating the second assumption. It is therefore unclear whether or not the density jump of a collisionless shock fulfills the MHD jump. Here we try to clarify this issue. A literature review is conducted on 68 articles dealing with Particle-In-Cell simulations of collisionless shocks. We analyze the factors triggering departure from the MHD density jump and quantify their influence on ΔRH, the relative departure from the Rankine–Hugoniot (RH) jump. For small departures we propose , where t is the timescale of the simulation, σ is the magnetization parameter and κ is a constant of order unity. The first term stems from the energy leakage into the accelerated particle. The second term stems from the downstream anisotropy triggered by the field (assuming an isotropic upstream). This relation allows us to assess to what extent a collisionless shock fulfills the RH density jump. In the strong field limit and for parallel shocks, the departure caused by the field saturates at a finite, negative value. For perpendicular shocks, the departure goes to zero at small and high σ's so that we find here a departure window. The results obtained have to be checked against full 3D simulations.

Development and calibration of a 1D thermo-fluid dynamic model of ventilation in tunnels

In complex, large civil infrastructures where ventilation has a crucial role for the safety of users in both normal operation and hazardous scenarios, the correct prediction of flow and heat transfer parameters is of fundamental importance. While full 3D simulation is applicable only to a limited extent, and the resort to 1D modeling is a common practice in both design and evaluation phases, the limitation of such models lies in the choice of transfer parameters, such as friction loss coefficients and heat transfer coefficients. In this work, an original approach based on the Finite Volume integration of the 1D flow and energy equations is presented. Such equations are to be solved on a network of ducts, representing the ventilation system in the 11.6 km long Mont Blanc Tunnel with a spatial resolution of 10 m. A preliminary calibration of a set of friction loss coefficients against a rich experimental dataset collected throughout a dedicated set of in situ tests is of particular concern here, as it is carried out by means of genetic optimization algorithms. Predictions of the flow field are in remarkable agreement with the experimental data, with an overall RMS error of ± 0.42 m/s. Further refinements and possible parameter choices are also discussed.

Effects of the ICE-T microphysics scheme in HARMONIE-AROME on estimated ice loads on transmission lines

In-cloud icing can cause damage to infrastructure and is challenging to forecast due to lack of a good representation of supercooled liquid water (SLW) in numerical weather prediction (NWP) models. We validate the new microphysics scheme, ICE-T, implemented into the NWP model HARMONIE-AROME, in full 3D simulations running over a 3 month period from December 1st 2016 to February 28th 2017. Output from the model simulations are first compared with conventional observations to evaluate the overall quality, and then used as input to an ice accretion model (IAM) and compared against measured ice loads at the two test sites Hardingnuten and Ålvikfjellet. The results show a clear shift towards more cloud water and snow, and less graupel and cloud ice. This shift leads to less precipitation along the coast and more inland. The estimated ice loads based on the cloud water from the simulations are generally increased. We also focus on two different icing events during January 9–18 and February 1–14. During the first event, both the run in its original configuration and the run with ICE-T overestimated the ice loads, while the second event was underestimated. For Ålvikfjellet ICE-T gives the best estimates, while for Hardingnuten the ice loads are overestimated when the wind direction is from the southeast. This is due to local terrain shielding not captured by the model. During the Feb 1–14 event, the wind direction was generally easterly, which makes comparison between the simulations and the observations more reliable. In this case, ICE-T gives a better ice load estimate. Although there are major uncertainties, especially concerning the number concentration of cloud droplets, and local terrain effects, ICE-T appears to give a better estimate of the ice loads.